1. Field of the Invention
This invention relates generally to a fuel cell system employing back-pressure control and, more particularly, to a fuel cell system that employs a discrete two-position valve at the cathode exhaust of the system fuel cell stack to control stack pressure and relative humidity.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The humidity or wetness of the membranes in a fuel cell stack is an important design criteria for effective stack operation. Too much water within the stack acts to prevent the oxygen in the cathode input gas from reaching the catalyst on the cathodes. Too little water within the stack causes the stack membranes to dry out and become more susceptible to cracking and other damage. The more current that the stack generates, the more water is generated as a by-product of the electro-chemical process. However, the more air that is forced through the stack by the compressor to provide more current, the more the stack membranes dry out. Typically, the stack has a 110% relative humidity during its most efficient operation. For 110% relative humidity, the exhaust gas is saturated 100%, and also includes a little bit of excess water.
Another factor that affects the stack relative humidity is stack temperature. As the stack temperature increases, the stack's ability to hold water in the vapor state also increases making it more difficult to maintain a desired stack relative humidity because more water is required to do so. Another factor that affects stack relative humidity is the stack pressure. As the pressure in the stack increases, the ability of the stack to hold water in the vapor state decreases. Thus, one of the most commonly used techniques to control cathode relative humidity is to control the fuel cell system pressure and temperature.
Fuel cell systems must reject waste heat. A fuel cell system will include a thermal coolant sub-system that removes heat from the stack so that it operates at its desired operating temperature. The heated coolant from the stack is directed to a radiator that reduces the temperature of the coolant so that it can be returned to the stack to remove the stack waste heat. The size of the radiator limits how much heat can be removed from the coolant.
The amount of waste heat that the coolant sub-system can reject is directly proportional to the operating temperature of the fuel cell system. If the system is able to operate at a higher temperature, a smaller radiator can be employed to remove the heat, thus conserving space. Unfortunately, a higher operating temperature requires a higher system pressure to keep the stack relative humidity at the desired level. In other words, as the temperature of the stack rises, its ability to hold water increases, thus requiring more water to meet the desired relative humidity. The higher pressure cancels the effect of the higher temperature on relative humidity. At higher operating temperatures, however, the stack may not produce enough water to meet the required relative humidity.
Because the size of the radiator is limited in a vehicle, a fuel cell system typically must operate at higher temperatures. Therefore, it becomes necessary to increase the pressure of the fuel cell stack so that more water is held therein to meet the desired relative humidity. However, high cathode pressures require larger amounts of compressor power, which results in a reduction in system efficiency.
Two approaches are known in the art to control fuel cell system pressure. One known approach is to employ a fixed orifice at the cathode exhaust output. Particularly, for high temperature applications the cathode output orifice is sized to provide a sufficient back-pressure to meet the relative humidity requirements at a maximum system temperature. However, the output orifice also causes high system pressure at low output power, and thus, the fuel cell system efficiency will suffer because of the higher compressor parasitic power.
For low temperature applications, where the thermal sub-system size is not critical, the cathode output orifice is sized to provide a nearly zero pressure drop. This allows the fuel cell system to run with low compressor parasitic losses, and is therefore efficient over the entire operating range. However, this fuel cell system will be large as a result of the large radiator required to reject the low-grade heat.
Modeling results have shown that a fixed orifice is capable of reducing the flow and pressure of the cathode exhaust gas without overloading the thermal sub-system. As a fuel cell power module decreases in flow and power, the amount of the waste heat the radiator has to dissipate will decrease. The amount of waste heat a radiator can dissipate is proportional to the temperature difference between the coolant and the ambient air. This value is often represented by Q/ITD, where Q is the waste heat and ITD is the initial temperature difference between the coolant and the air. Radiators are sized for a maximum Q/ITD at maximum power. Therefore, as the fuel cell turns down in flow and power, the waste heat requirement is lowered, which allows the operating temperature to be lowered. This allows the operating pressure to be lowered.
FIG. 1 is a graph with cathode input air flow on the horizontal axis and the required compressor pressure on the vertical axis showing the possible compressor delivery pressure for a fuel cell system employing only a fixed orifice to control system pressure. Graph line 40 shows the operation curve of the system for flow versus pressure for a high pressure drop at the fixed orifice, and graph line 42 shows the operation curve for flow versus pressure for a near zero pressure drop at the fixed output orifice. The fuel cell system will operate on one of the graph lines 40 or 42, regardless of the system operating temperature. The high pressure graph line 40 wastes energy at part power conditions, and the low pressure graph line 42 can only meet humidity requirements at low temperatures.
Another known approach is to use active cathode pressure control. This has been done in the past with high-resolution control valves. These high-resolution control valves offer many discrete valve open positions or an analog control where fluid flow through the valve can be set at any desirable location. The position of the valve is determined by the current operating temperature of the system and the amount of water that is being generated by the stack to provide a calculation of what pressure is necessary to provide a 110% stack relative humidity. Further, it is necessary to provide a safety device that prevents the valve from failing in the closed position, which could cause catastrophic stack failure from high pressure. While this approach offers a good solution that attempts to optimize system efficiency over its entire operating range, the control valve and support software are high cost components. Further, in the majority of the operating conditions of a fuel cell system, this level of control is unnecessary.
FIG. 2 is a graph with cathode input air flow on the horizontal axis and the required compressor pressure on the vertical axis showing the possible compressor delivery pressures for a fuel cell system employing a high resolution back-pressure valve. Graph line 44 shows how the cathode pressure can be directly controlled independent of flow to provide the desired relative humidity. At a constant low temperature operation, the control valve is wide open so that the pressure drop at the cathode exhaust is near zero. As the flow increases, the system temperature increases, and the valve will be systematically closed to provide the desired fuel cell back-pressure to control the relative humidity.
For fuel cell systems having a high resolution back-pressure control valve, the valve position is changed as a function of coolant temperature in order to maintain the desired relative humidity. However, the pressure drop across a fixed orifice is a function of the fluid velocity, fluid viscosity and orifice shape. Because the orifice shape cannot be changed, the cathode pressure and flow rate cannot be independently controlled. If the fixed orifice is sized to meet the humidity requirements at the maximum temperature, maximum flow and maximum Q/ITD point, it will provide sufficient humidity control in turn down. The minimum pressure required to maintain the desired humidity is given by operating at the minimum temperature possible, while maintaining a maximum Q/ITD condition in turn down. This required pressure is less than the pressure provided by the pressure drop across a fixed orifice in turn down when sized at the maximum point.
A system model was constructed where the maximum allowable Q/ITD was set at maximum power and flow. As the module turned down in flow and power, the coolant temperature was lowered only as much as possible without exceeding the maximum Q/ITD. The system back-pressure requirement was then calculated in order to meet the required humidity level. This gives a flow versus pressure drop curve that is required of the back-pressure valve in order not to exceed the maximum Q/ITD limit. FIG. 3 is a graph with cathode input air flow on the horizontal axis and back-pressure valve pressure drop on the vertical axis, where graph line 46 shows this relationship.
The model was re-run using a fixed area orifice for back-pressure control. The orifice was modeled based on a sharp edge orifice that was sized to meet the desired humidity at the maximum flow and temperature condition. The pressure drop through a sharp edge orifice is proportional to the square of the fluid velocity. Under this assumption, a flow versus pressure drop curve was generated for the system using the fixed area orifice, as shown by graph line 48 in FIG. 3. This modeling shows that the pressure drop for the fixed area orifice meets or exceeds the pressure drop requirements to achieve the desired humidity at steady state turn down points. The fixed area orifice will cause the fuel cell to be slightly over pressurized at part power conditions, which will give a slightly higher than optimal humidity. This will cause a slight system efficiency hit, but is not damaging to stack durability. If the fixed orifice pressure drop was less than the pressure drop required to meet the Q/ITD limit, then the humidity would be lower than desired and the stack may be damaged.